您现在的位置: 纽约时报中英文网 >> 纽约时报中英文版 >> 科学 >> 正文


更新时间:2017-12-24 12:41:31 来源:纽约时报中文网 作者:佚名

The ambitious plan to stop the ground from sinking

From Miami to Jakarta, coastal communities around the world are battling the effects of sea level rise.


But in some places, the problem is exacerbated by another phenomenon: the land is falling.


The Indonesian capital, for example, is sinking up to 17cm (6.7 inches) per year. “That’s an issue, because they’re right at sea level,” says Michelle Sneed, a land subsidence specialist at the US Geological Survey (USGS). “They have this added pressure of increased flooding and sea level rise. They built seawalls. But the city is subsiding so quickly that, at high tides, water just pours over.”

以印度尼西亚首都雅加达为例,该地地面每年沉降最高可达17厘米。美国地质调查局(US Geological Survey)的土地沉降专家米歇尔·斯尼德(Michelle Sneed)说:"因为这些地方基本上与海平面持平,沉降问题增加了洪涝和海平面上升带来的压力。当地人们正在建造海堤,但是由于这座城市正在快速沉陷,因此涨潮时,海水会直接漫过海堤。"

Partly because of examples like Jakarta, subsidence often gets misinterpreted by climate change sceptics, who argue this phenomenon alone explains increased flooding in coastal areas. The reality is more challenging. Both sea level rise and subsidence are happening at once. But while sea level rise is a global issue caused by the warming of the oceans and melting of the world’s ice caps, land subsidence is a local problem, affecting some communities but not others.


In coastal areas unlucky enough to be hit by both phenomena, the risk of flooding can be severe. And although inland communities are unlikely to suffer much from sea level rise, many, including Mexico City and California’s San Joaquin Valley, are grappling with the challenges posed by sinking land instead.

在不幸同时遇到两种现象的沿海地区,洪涝的风险会大大增加。虽然内陆地区不太可能受到海平面上升的影响,但是像墨西哥城和加州的圣华金谷(San Joaquin Valley)等很多地区正面临地面沉陷的挑战。

But there is good news. While most scientists agree that rising sea levels only can be mitigated by lowering carbon emissions, which will require global consensus, communities can take control of their own land subsidence.


“If water is high because of sea level rise, then you have to address the entire world,” says University of Utrecht geologist and subsidence researcher Gilles Erkens. “In some ways, that makes it easier to address land subsidence, because you only have to look at a local area.”

乌得勒支大学(University of Utrecht)的地质学家和地面沉降问题研究员吉尔·厄肯斯(Gilles Erkens)说:"如果水位高是因为海平面升高,那就是一个全球问题。这在某种程度意味着降低了应对土地沉降问题的难度,因为你只需要研究和解决本地的情况。"

Some cities, including Shanghai and Tokyo, have already solved the problem. Other communities, like the Hampton Roads area of eastern Virginia, are now coming up with their own creative solutions.

上海、东京等城市已经解决了地面沉降问题。像弗吉尼亚州东部的汉普顿锚地(Hampton Roads)等地区正在提出自己的创新解决方案。

If you’re surprised to hear that subsidence is a localised (or solvable) problem, you may be thinking of one kind of vertical land motion: global isostatic adjustment (GIA). A hangover from the last Ice Age that occurred around 12,000 years ago, GIA is the rebound of Northern Hemisphere land after it was freed of the weight of billions of tonnes of ice. While the areas once under the now-melted ice are rising, those on the edges are falling in response.

当你听说地面下沉是一个地区性问题(或可以解决的问题)时,如果你感到惊讶,不妨设想一下土地的垂直运动:全球均衡调整(GIA)。在大约12,000年前的冰河世纪(Ice Age),全球均衡调整的结果就是北半球从数十亿吨重的冰川解脱出来后出现的抬升。随着冰川的融化,冰川下的陆地开始升高,冰川边缘的陆地正在相应的下沉。

Imagine poking a balloon with your finger. You’d create an indent, as well as a bulge around your fingertip. When you release your finger – much like melting the ice – the indent bounces back while the bulge goes down. In North America, the spot with the indent is Canada and Alaska, while the bulge is the mid-Atlantic.


Stretch that millisecond-long experiment over millennia and you have something similar to what is happening to the Earth right now. But as you might expect, GIA is a relatively slow process and should not be confused with the more dramatic subsidence affecting some communities.


“If a localised area is seeing its land sinking significantly, it’s not because of global isostatic adjustment,” says NOAA’s Philippe Hensel. “At a maximum, global isostatic adjustment is going to be pretty minor.”

美国国家海洋和大气管理局(NOAA)的菲利普·亨塞尔(Philippe Hensel)说:"如果一个地区发现地面大幅下沉,那原因就不是全球均衡调整。全球均衡调整最多也只能造成微小的影响。"

The most significant increase in height due to GIA, in places like Alaska and Canada, is nearly 10mm per year, says Hensel. But those areas that are moving downwards because of GIA are falling by a maximum of 1mm or 2mm annually.


Water slide


For most communities around the world, therefore, the reason for significant subsidence is something entirely man-made: groundwater extraction.


“Everything you extract from the underground results in subsidence,” says geologist Simone Fiaschi, who studies subsidence at the University of Padova. “You remove something from the layers the terrain is made of, so the ground starts to collapse.”

帕多瓦大学(University of Padova)的地面沉降学者西蒙·菲亚斯基(Simone Fiaschi)说:"每次从地下挖掘东西出来都会造成地面下沉。随着人们挖走地下的东西,地面就会开始坍陷。"

That means other types of extraction, such as removing methane gas or oil, can also produce the same effect. But groundwater, one of the most important sources of fresh water supplies in the world, is usually the culprit. In India, the world’s largest user of groundwater, 85% of drinking water comes from the ground; in Europe, 75% of the population gets drinking water from groundwater.


And then there are all of the other uses. In the US, for example, irrigation for agriculture accounted for the withdrawal of 225 million cubic metres (49.5 billion gallons) of groundwater per day in 2010 – 60% of the total removed from the ground. That’s enough water to fill up an empty Lake Tahoe, California’s largest lake by volume, in less than two years.

地下水还有其他很多用途。以美国为例,2010年农业灌溉每天抽取2.25亿立方米地下水——占地下水总抽取量的60%。其水量足以在不到两年的时间里填满加利福尼亚藏水量最大的湖泊——太浩湖(Lake Tahoe)。

Groundwater is supposed to replenish itself naturally from rain and snowfall seeping through the rock. In many parts of the world, though, the ground is being emptied of water faster than it has time to recharge. This can lead to depletion of the water table and dwindling water supplies – but also can cause the soil to compact so that the layers on top drop, sometimes significantly.


Mexico City, for example, depends upon a local aquifer for about half of its potable water. Thanks to a combination of the city’s vast population of 21 million people and inefficient water use – 42% is lost to leakages – the aquifer is being overdrawn. At this rate, it will be empty within 50 years, says Arnoldo Matus Kramer, the city’s chief resilience officer. In the meantime, parts of the city are sinking by 30cm (12 inches) per year.

以墨西哥城为例,其饮用水大约一半都依赖本地的地下水含水层。这座城市2100万的庞大人口规模和低效的用水方式——42%都通过泄漏流失了——含水层已经被过度抽取。墨西哥城的首席地下水恢复官阿诺尔多·马图斯·克雷默(Arnoldo Matus Kramer)表示,按照目前的速度,50年后该含水层就会枯竭。与此同时,该城市的一些区域每年将下沉高达30厘米。

As a result, the city is trapped in a vicious cycle where subsidence damages the water pipe infrastructure and makes it more difficult to maintain, which leads to more leaks and more water being withdrawn. And as well as making the city more vulnerable to water shortages, subsidence also may have made some buildings more vulnerable to Mexico City’s recent earthquake, says Kramer.


Exactly how much of the world is affected by subsidence is hard to say. “We’re still trying to get the data for places around the world,” says Erkens. “For many places, we don’t know exactly what’s happening, which also hampers our options for dealing with the challenges.”


Still, from the data available, scientists agree that they’ve seen something promising. Stopping groundwater pumping can halt subsidence – and even help the land rebound.


Cities have proven it before. After decades of groundwater extraction in Tokyo, the land began to sink more and more, peaking in 1968 at 24cm (9 inches) per year. At around the same time groundwater pumping in the city also reached a high of 1.5 million cubic meters (329 million gallons) per day. In response, Tokyo’s government passed laws limiting pumping. By the early 2000s, the city’s subsidence slowed to 1cm (0.4 inches) a year.


But halting pumping requires changing the main source of a city’s water. And for some areas, that may not be possible. The San Joaquin Valley, which spreads across some 25,900 square km (10,000 square miles) in the centre of California, relies on groundwater for its main industry – agriculture. Exacerbated by the recent drought, parts of the region have begun to sink by up to 60cm (2ft) per year.


“That’s among the very fastest in the world,” says the USGS’s Sneed, who is based in California. Worsening the problem has been a recent change toward more water-intensive practices as agriculture has shifted from rotation crops like tomatoes and peppers to permanent crops like orchards and vineyards.


Although subsidence here isn’t causing flooding, it’s still undermining the area’s infrastructure. One example is its massive canal system, which it uses to move water around the area. Parts of the valley are subsiding at different rates, causing the gravity-reliant canal system to fail. As a result, California legislators signed a law in 2014 to ensure that groundwater use doesn’t cause unreasonable land subsidence.


How that will be done, though, has yet to be determined. Relying on alternative water sources seems unlikely, according to Sneed, as California doesn’t have much capacity for further reservoirs.


“I think they’re starting to realise what a monumental task it will be,” Sneed says. “Locals are going to have make some very hard choices that they haven’t had to make before on how they use their land.”


Pumped up


One way cities like Shanghai have tackled the problem is by not only limiting pumping, but by recharging their aquifers. An even more creative solution, though, is being developed in Virginia.


There, the southern part of the Chesapeake Bay region, known as Hampton Roads, is threatened by three different forces. This is an area at the edge of the now-melted ice – so while GIA here is around 1mm per year, that rate of drop is still among the fastest in the world. The second problem is sea level rise, contributing around another 2mm per year.

切萨皮克湾(Chesapeake Bay)南部的汉普顿锚地正受到三股不同力量的威胁。该区域位于冰川融化区域的边缘——虽然全球均衡调整在这里的影响大约是每年1毫米,但是地面的下沉速度仍然排在全世界前列。第二个问题是海平面的上升,造成每年地面下沉2毫米。

But at about 2.8mm per year, the single biggest contributor is groundwater pumping from the massive Potomac aquifer.


In an area this flat, those millimetres add up. The area suffers frequent flooding, as well as saltwater intrusion into both the aquifer and into delicate wetlands that are in danger of being inundated.


Ted Henifin is the general manager of the Hampton Roads Sanitation District. A few years ago, his team started to wonder if there was a better use for the wastewater that they were processing and dumping into the mouth of the Chesapeake. “It’s not like the water we put back into the waterways is used by anyone else, or even needed,” he says.

汉普顿锚地卫生区(Hampton Roads Sanitation District)的总经理泰德·海尼芬(Ted Henifin)在几年前曾带领团队研究废水是否有更好的利用方法。他们当时正在对废水进行处理并排入切萨皮克湾的入海口。他说:"我们排出的废水没有人会用,甚至没有人需要。"

So what if they could use the water for something of value? That train of thought led to an innovative new project called Swift. Instead of dumping the water, the project will treat the wastewater – which totals some 682,000 cubic metres (150 million gallons) each day – to standards that will meet that of drinking water. It will also be given the exact same profile, including the salinity, as the groundwater. Once it’s been processed, the water will be pumped back into the aquifer.


The project is still starting out, with the goal of getting permits in 2019 and injecting 45,500-91,000 cubic metres (10-20 million gallons) a day by 2023. But the models have already found that adding the water can increase pressures as far as Maryland and North Carolina.

该项目目前还处于起步阶段,目标是在2019年取得许可证,在2023年之前每天向含水层注入45,500至91,000立方米水。但是模型已经发现增加注水有可能增加远及马里兰州(Maryland)和北卡罗来纳州(North Carolina)的水压。

“With the total aquifer compaction that would have been seen without our project, if we continue with the permitted withdrawals that we’ve got to the end of a 50-year period, there’s a total compaction, in the worst areas, of about 2ft (61cm),” says Henifin. “If the same model is run with our water going in, we eliminate that entirely.”


If the project works, the plan is to scale up to full capacity, 545,000 cubic metres (120 million gallons) a day, by 2030 – and then replicate the programme in wastewater treatment plants across other counties.


David Nelms, a USGS scientist who has been involved in the project, cautions that it may not be a panacea. As elsewhere, the ground here is layered with both clay and sand. Extraction over the last century has compacted both layers. When water is injected, though, it will ‘pump up’ only the sand. The clay remains compacted. “You will never get that back,” says Nelms. “That’s permanent. But [the project’s] sites are scattered and the geology is different in each one of them, so you should expect different responses in different places.”

参与该项目的美国地质调查局(USGS)的科学家大卫·内尔姆斯(David Nelms)提醒这可能不是万灵药。和其他地方一样,这里的土地也是由粘土层和沙层构成的。过去一个世纪的开采已经导致这两个土层开始萎缩。在注水时,只能让沙子吸水,而粘土仍然是萎缩状态。内尔姆斯说:"那是无法恢复的,是永久的。但是项目的地点很分散,每个地方的地质构造不一样,所以在不同的地方要采取不同的应对方式。"

Land subsidence may be a complicated problem. But with projects like Swift aimed at tackling it, there may be reason for optimism – not only to fix subsidence, but to mitigate its twin challenge of sea level rise. “In terms of can we do something about sea level in our lifetime, this idea may be the only one we’ve come up with that may buy some time for our region,” Henifin says.